Process for reconditioning pistons



Patented Oct. 18, 1938 UNITED STATES PATENT OFFICE No Drawing.Application October 5, 1937, Serial No. 167,473

7 Claims.

This invention relates to a process for reconditioning pistons and moreparticularly to processes for reconditioning the worn pistons ofinternal combustion engines by stretching, expanding,

or ovalizing them, so that they will fit their cylinders with properoperating clearance.

It is well known that when an internal combustion engine piston has beensubjected to alternately hot and cold temperatures as a phase of 10operation of the engine, the metal of the piston in them becomesseasoned and tempered. Piston metal seasoned in this manner is morewear-resisting and suitable than the material of a new piston. This isdue to the fact that operation for long periods of time results inmaking the piston surface very smooth and closing the pores of themetal. By the time a piston has become thus seasoned and the poresclosed, it has worn down to such an extent that there is too muchclearance between the piston and the engine cylinder walls for properand efficient operation, excessive clear ance resulting in piston slap,excessive oil consumption and blow-by. Thus, when the metal of a pistonhas attained its most efficient characteristics, the piston itself hasbecome too small.

In the reconditioning of motors after the pistons and cylinder wallshave worn, it has been common to install new pistons of enlargeddiameter, but this procedure does not take advantage of the improvedcondition of the metal of the old piston.

Another method heretofore followed in some instances has been to insertdevices within the old piston having a tendency to exert an expandingforce particularly on the split skirt of the piston,

so as to spread the skirt and enlarge the diameter to take up excessiveclearance. It has been found, however, that the inserted expandingdevice produces only a temporary correction, since in a short time, itloses its expansive force, permitting 40 the piston to return to itsnormal diameter as reduced by wear. Continued flexing at hightemperatures contributes to the early failure of such expanding devices.

Another method which has been followed with only some degree of successin the past is to stretch the piston on an expanding or stretchingmachine, but insofar as I am aware, piston expanding machines andmethods heretofore known have not produced uniformly satisfactoryresults, one very common difficulty having been the crystallization orother disturbing of the piston metal, resulting in subsequent crackingor breaking of the pistons.

In accordance with my invention, worn pistons are treated and expandedin a novel manner, so as to give the metal a permanent set to anenlarged diameter, without harmfully disturbing the crystallinestructure. The advantages of thus using the old piston are many, themost important being that the piston metal being seasoned and the poresbeing already closed, the pistons will be less subject to wear than evenentirely new pistons. A further advantage is that the using of the oldpistons eliminates the necessity of providing new piston pins andfurthermore, of course, the 10 cost of expanding the pistons is verysmall as compared to the cost of new pistons. Moreover, when using newpistons, it is necessary to supply and install new piston pins, whereasby reconditioning old pistons, this substantial added 15 expense iseliminated.

It is an object of the present invention to provide a method forexpanding pistons in such a manner as to stretch the piston to thedesired increased diametral dimension and to cause the metal to setpermanently to such increased dimension without, however, disturbing theinternal metal structure and without rendering the piston susceptible orsubject to cracks or breakage.

Generally stated, a process in accordance with 25 my invention includesthe steps of heating the piston to a suitable temperature, then while itis hot, stretching the piston transversely of its axis and thereaftercooling the piston slowly, so as to not set up any internal stresses. Ihave found that the temperatures to which the piston is heated at thetime during which the stretching forces apply and the rate of coolingmust be controlled quite carefully in order to obtain the best results.I 35 The invention will be more fully understood from a consideration ofthe control exercised over the process as set forth below, it beingunderstood, however, that although the temperatures, stretching times,and cooling rates are important and must be controlled with care, thetemperatures and so forth stated are given. by way of example and aresubject to reasonable variation within the limits indicated. As a resultof bothlaboratory tests and the successful commercial reconditioning ofmany thousands of pistons, I have found that the pistons first should beheated in any suitable oven or the like to a temperature of betweenapproximately 575 F. to 700 F. for a period of about one-half 50 hour. Asomewhat longer heating period may be employed without harmful results,but is usually unnecessary. The pistons should, however, be maintainedat the temperature indicated for a period of about one-half hour and itis immaterial whether the pistons are put in a cool oven and brought upto temperature with the oven, or whether they are put in an oven alreadyat a temperature of from about 575 F. to about 700 F.

After the pistons have been maintained at the temperature indicated forthis period, they are removed from the oven and immediately placed on asuitable stretching apparatus, preferably such as is disclosed andclaimed in my copending application, Serial No. 118,964. The process maybe employed for reconditioning various types of pistons, whether of thesplit-skirt or non-splitskirt type. When treating split skirt pistons,they should be placed on the stretching apparatus in such a manner thatthe stretching force is applied at diametrically opposite pointsrespectively mid-way between the splits in the skirts. The stretchingforce is then applied against the insides of the piston skirts until thepistons have beenexpanded or stretched transversely of their axes to thedesired diameter as indicated by a suitable gauge or the like. Afterthey have been stretched to the desired increased diameter, the pistonsare allowed to remain on the stretching apparatus for about twentyseconds to permit the metal to set. The pistons are then removedfrom thestretching apparatus and placed so as to be well protected from draftsor cold air, so that they will cool gradually to about 250 F. Theyshould be cooled at such a rate that they will reach 250 F. within aboutfifteen minutes, and usually this will require a cooling rate of notmore than about 15 per minute. This slow cooling may be caused to takeplace in a slightly heated oven or in an enclosure which need not beheated by any source other than the pistons themselves, but which is soconstructed as to protect the pistons from drafts or the like and tocause them to cool at the desired slow rate. After the pistons havereached a temperature of about 250 F., they may be permitted to coolmore rapidly without harmful results, thus making it possible to conductthe process more rapidly. Experience has shown that by following myimproved process, the pistons retain their desired characteristics,including their smooth surfacewith closed pores and their ability toresist wear. Actually, therefore, pistons reconditioned in accordancewith my process are superior to new pistons, and it is much moreeconomical to recondition the old pistons than it is to provide newpistons.

The process described above by applying the stretching force midwaybetween the splits of a split skirt piston expands the piston to asomewhat ovalized shape, which is desirable.

The method described above has been found to produce the mostsatisfactory results in the reconditioning of hundreds of pistons and isthe at present preferred method in accordance with my invention, but itwill be understood that various changes may be resorted to withoutdeparting from the invention as defined in the claims.

I claim:

1. A process for expanding pistons comprising heating the piston andmaintaining it at a temperature of from 575 to 700 degrees Fahrenheitfor about one-half hour; stretching it by external force appliedtransversely of its axis while hot; and then cooling it substantiallywithout disturbing its crystalline structure and initially at theapproximate average rate of about 15 degrees Fahrenheit per minute untilit reaches a temperature of about 250 degrees Fahrenheit.

2. A process for expanding pistons comprising heating the'piston andmaintaining it at a temperature of from 575 to 700 degrees Fahrenheitfor about one-half hour; stretching it by external 'force appliedtransversely of its axis while hot;

and then cooling it at the approximate average rate of about 15 degreesFahrenheit per minute until it reaches a temperature of about 250degrees Fahrenheit, and then cooling it more rapidly to roomtemperature.

3. A process for expanding pistons comprising heating the piston andmaintaining it at a temperature of from about 5'75 to 700 degrees Fahrenheit; stretching it by external force applied transversely of its axiswhile hot; and then cooling it substantially without disturbing itscrystalline structure at the approximate average rate of about 15degrees Fahrenheit per minute.

4. A process for expanding pistons comprising heating the piston andmaintaining it at a temperature of from about 575 to 700 degreesFahrenheit; stretching it by external force applied transversely of itsaxis while hot and for about twenty seconds; then cooling itsubstantially without disturbing its crystalline structure and initiallyat a rate slower than that at which it would cool in ordinary room air;and then cooling it in air at room temperature.

5. A process for reconditioning split-skirt pistons comprising heatingthe piston for about onehalf hour at a temperature of from about 575degrees Fahrenheit to about 700 degrees Fahrenheit, applying stretchingpressure for about twenty seconds at opposed points on opposite sidessubstantially midway between the splits in the skirt; and cooling thepiston substantially without disturbing its crystalline structure andinitially to a temperature of approximately 250 degrees Fahrenheit inabout fifteen minutes.

6. A process for expanding pistons comprising heating the piston andmaintaining it at a temperature from 575 to 700 degrees Fahrenheit;stretching it while hot by external force applied interiorly only atdiametrically opposite points to a predetermined extent; and cooling thepiston while'it is stretched to said predetermined extent substantiallywithout disturbing its crystalline structure.

'7. A process for expanding pistons comprising heating the piston andmaintaining it at a temperature from 5'75 to 700 degrees Fahrenheit forabout one-half hour; stretching it while hot by external force appliedinteriorly only at diametrically opposite points to a predeterminedextent; and cooling the piston while it is stretched to saidpredetermined extent substantially without disturbing its crystallinestructure.

DAVID RUBIN.

